A sad chapter in National Football League history was written last week when the Colts fled Baltimore in the middle of the night. The once-proud NFL city was dumped for a new suitor, Indianapolis, and the club obviously wanted to slip out of town on cat's paws, not horse's hooves.

Interest in the team had declined and attendance fallen off since the Johnny Unitas heyday of the late 1950s and '60s. But, of course, the product hasn't been much lately, with nary a winning season in the last six. In a show of good faith, though, fans responded this past season when the team posted an encouraging 7-9 record, a big improvement on the winless, strike-shortened 1982 campaign. The team sold an average of almost 41,000 tickets per game and attracted a regular season record 60,559 for one game.

Still, there has been a fair amount of turbulence within the franchise and with its dealings with the city.

Indianapolis has been itching to get an NFL team to occupy the 60,000-seat Hoosier Dome, which is nearing completion. And rather than wait until the league next expands, Indianapolis basically outbid Phoenix to get the ''free agent'' Colts, promising $7 million in ticket sales for the next 12 years and providing an attractive stadium rental agreement. Phoenix couldn't match the offer, nor could Baltimore, which once had to sell 15,000 season tickets to get the team in 1953.

Sports-minded Indianapolis is deserving of its own team and should make a good home for the Colts. Distressing, however, is the classless manner in which the move was carried out and Indianapolis's willingness to act as an accomplice in the getaway. Baltimore and the state of Maryland intend to fight for the club's return. Timely play of goalie Billy Smith

A hot goaltender may be the single most important factor to success in the National Hockey League playoffs. Billy Smith, then, couldn't have picked a better time to elevate his game.

The New York Islander netminder was named the NHL's Player of the Week right before the start of the Islanders' current best-of-five playoff series with the New York Rangers. The club is seeking its fifth consecutive Stanley Cup championship, which would tie the record set by the Montreal Canadiens from 1955 -56 through 1959-60. The Canadiens never had to play more than two playoff rounds in their championship years. The Islanders, meanwhile, have been winning under today's four-round system.

Smith, a ''money'' goalie, consistently rises to the occasion. Last year he was named the most valuable player in the Stanley Cup final when he held Edmonton to six goals in New York's four-game sweep. Touching other bases

* Fans of the New York Mets have grown accustomed to seeing their team lose, but not on opening day. Before Monday's 8-1 defeat to Cincinnati, the Mets had won their last nine openers, just one short of a record.

* Under the direction of imaginative president Fred Lebow, the New York Road Runners Club has sponsored everything from five-borough marathons and Fifth Avenue miles to fun runs around Central Park. The club's latest project was bringing the world cross-country championships to the western hemisphere for the first time. The event was hosted at New Jersey's Meadowlands Race Track, where man-made hills and hay bale obstacles were added to the infield area. Some 17, 000 spectatators watched the races - men's, women's, and junior men's - from the grandstand. The respective winners were Portugal's Carlos Lopes, Romania's Maricica Puica, and Spain's Pedro Casacuberta.

* Viewers already into outdoor recreation, and even those contemplating armchair escapes may find a new PBS television series to their liking. ''The Great Outdoors,'' which debuts this Saturday at l and 7:30 p.m. (check local listings), attempts to ''demystify the outdoors experience'' in 13 half-hour segments. Host Jim Tabor, a noted outdoors writer, will serve as the resident guide as the show samples backpacking, canoeing, cycling, and windsurfing among other pursuits, while mixing in how-to tips on getting started, buying gear, and using the proper technique.